thumb|The 1962 "[[Sedan (nuclear test)|Sedan" plowshares shot displaced 12 million tons of earth and created a crater deep and wide.]]
Project Plowshare was the overall United States program for the development of techniques to use nuclear explosives and large non-nuclear explosions for peaceful construction purposes. The program was organized in June 1957 as part of the worldwide Atoms for Peace efforts. As part of the program, 35 nuclear warheads were detonated in 27 separate tests. A similar program was carried out in the Soviet Union under the name Nuclear Explosions for the National Economy, although the Soviet program consisted of 124 tests.
Successful demonstrations of non-combat uses for nuclear explosives include rock blasting, stimulation of tight gas, chemical element manufacture, unlocking some of the mysteries of the R-process of stellar nucleosynthesis and probing the composition of the Earth's deep crust, creating reflection seismology vibroseis data which has helped geologists and follow-on mining company prospecting.
The project's uncharacteristically large and atmospherically vented Sedan nuclear test also led geologists to determine that Barringer crater was formed as a result of a meteor impact and not from a volcanic eruption, as had earlier been assumed. This became the first crater on Earth definitely proven to be from an impact event.
Negative impacts from Project Plowshare's tests generated significant public opposition, which eventually led to the program's termination in 1977. and the deposition of fallout from radioactive material being injected into the atmosphere before underground testing was mandated by treaty.
Rationale
By exploiting the peaceful uses of the "friendly atom" in medical applications, earth removal, and later in nuclear power plants, the nuclear industry and government sought to allay public fears about nuclear technology and promote the acceptance of nuclear weapons. At the peak of the Atomic Age, the United States Federal government initiated Project Plowshare, involving "peaceful nuclear explosions". The United States Atomic Energy Commission chairman at the time, Lewis Strauss, announced that the Plowshares project was intended to "highlight the peaceful applications of nuclear explosive devices and thereby create a climate of world opinion that is more favorable to weapons development and tests". These tests were to demonstrate that atomic bombs can be used for peaceful purposes, that the atomic sword could be beaten into a plowshare.
Proposals
thumb|One of the Chariot schemes involved chaining five thermonuclear devices to create an artificial harbor.
thumb|Plowshare's proposed canals near the Panama Canal.
Proposed uses for nuclear explosives under Project Plowshare included widening the Panama Canal, constructing a new sea-level waterway through Nicaragua nicknamed the Pan-Atomic Canal, cutting paths through mountainous areas for highways, and connecting inland river systems. Other proposals involved blasting caverns for water, natural gas, and petroleum storage. Serious consideration was also given to using these explosives for various mining operations. One proposal suggested using nuclear blasts to connect underground aquifers in Arizona. Another plan involved surface blasting on the western slope of California's Sacramento Valley for a water transport project.
Project Carryall, proposed in 1963 by the Atomic Energy Commission, the California Division of Highways (now Caltrans), and the Santa Fe Railway, would have used 22 nuclear explosions to excavate a massive roadcut through the Bristol Mountains in the Mojave Desert, to accommodate construction of Interstate 40 and a new rail line.
At the end of the program, a major objective was to develop nuclear explosives, and blast techniques, for stimulating the flow of natural gas in "tight" underground reservoir formations. In the 1960s, a proposal was suggested for a modified in situ shale oil extraction process which involved creation of a rubble chimney (a zone in the oil shale formation created by breaking the rock into fragments) using a nuclear explosive. However, this approach was abandoned for a number of technical reasons.
Nuclear weapon designs
For gas stimulation shots, one problem was extreme conditions. They needed a device with a small diameter on the order of that could withstand extended periods at without refrigeration. Another major problem was "residual tritium". The Rulison shot used an expensive fission explosive. The Miniata shot tested a cheaper and smaller (9 inch diameter) "minimum residual tritium" design device known as Diamond. Notably, Los Alamos Scientific Laboratory also proposed a 9 inch "tritium free" device that would use proton-boron fusion to strongly limit the neutron radiation produced.
Plowshare testing
The first Peaceful Nuclear Explosion (PNE) blast was Project Gnome, conducted on December 10, 1961, in a salt bed southeast of Carlsbad, in southeast New Mexico. The explosion released 3.1 kilotons (13 TJ) of energy yield at a depth of which resulted in the formation of a diameter, high cavity. The test had many objectives, the most public of which involved the generation of steam which could then be used to generate electricity. Another objective was the production of useful radioisotopes and their recovery. Yet another experiment involved neutron time-of-flight physics, and a fourth experiment involved geophysical studies based upon the timed seismic source. Only the last objective was considered a complete success. The blast unintentionally vented radioactive steam while the press watched. The partly developed Project Coach detonation experiment that was to follow adjacent to the Gnome test was then canceled.
A number of proof-of-concept cratering blasts were conducted; including the Buggy shot of five 1-kiloton devices for a channel/trench in Area 30 and the largest being 104 kiloton (435 terajoule) on July 6, 1962, at the north end of Yucca Flats, within the Atomic Energy Commission's Nevada Test Site (NTS) in southern Nevada. The shot, "Sedan", displaced more than of soil and resulted in a radioactive cloud that rose to an altitude of . The radioactive dust plume headed northeast and then east towards the Mississippi River. and CER Geonuclear Corporation for the Rio Blanco test.
The final PNE blast took place on May 17, 1973, under Fawn Creek, north of Grand Junction, Colorado. Three 30-kiloton detonations took place simultaneously at depths of . If it had been successful, plans called for the use of hundreds of specialized nuclear explosives in the western Rockies gas fields. The previous two tests had indicated that the produced natural gas would be too radioactive for safe use; the Rio Blanco test found that the three blast cavities had not connected as hoped, and the resulting gas still contained unacceptable levels of radionuclides.
By 1974, approximately $82 million had been invested in the nuclear gas stimulation technology program. It was estimated that even after 25 years of production of all the natural gas deemed recoverable, only 15 to 40% of the investment would be recouped. Also, the concept that stove burners in California might soon emit trace amounts of blast radionuclides into family homes did not sit well with the general public. The contaminated gas was never channeled into commercial supply lines.
The situation remained so for the next three decades, but a resurgence in Colorado Western slope natural gas drilling has brought resource development closer and closer to the original underground detonations. By mid-2009, 84 drilling permits had been issued within a radius, with 11 permits within mile of the site.
Impacts, opposition and economics
Operation Plowshare "started with great expectations and high hopes". Planners believed that the projects could be completed safely, but there was less confidence that they could be completed more economically than conventional methods. Moreover, there was insufficient public and Congressional support for the projects. Projects Chariot and Coach were two examples where technical problems and environmental concerns prompted further feasibility studies which took several years, and each project was eventually canceled.